Data

There are two fundamental data types: Pairs and Atoms (not-pairs). Atoms come in three variants: Symbols, Closures (functions), and some Special Symbols.

Symbols

Symbols are unique names and consist of any combination of maximum 16 of the following characters:

a b c d e f g h i j k l m n o p q r s t u v w x y z 1 2 3 4 5 6 7 8 9 0 - . ? _

where . cannot be the leading character.

Additionally, any printable ASCII character can be part of a symbol when prefixed with the escape character \ (backslash), with the exception of (, ), ', and $.

This\ is\ a\ sym\!

Furthermore, uppercase letters are accepted but converted to lowercase unless the character is escaped.

Special Symbols

There are some predefined special symbols managed by SLED, for example nil which means "empty list". See the index for details.

Quote

A quote means "do not evaluate". Via the quote special form a symbol is registered:

(quote sym)

For convenience the ' short form syntax may be used:

'sym

Essentially quoting declares something as data instead of code.

Pairs

Pairs consist of a head and a tail, each holding either an atom or another pair. A pair can be created as data using the . (dot):

'(a . x)

or as result of the cons builtin function:

(cons 'a 'x)

Pair elements (head and tail) are immutable.

Lists

A list is a sequence of pairs where each tail points to a distinct other pair except one (the last) whose tail is the nil value, which is equivalent to (). Here are some lists:

nil
'()
'(a . nil)
'(a . (b . nil))

A list can be created as data also by:

'(a b)

or as result of the (standard library) list function:

(list 'a 'b)

Improper List

An improper list does not terminate by nil, for example:

(a . (b . (c . d)))

Association List

An association list is a list where each element is a pair (association):

((a . x) (b . y) (c . z))

The head part of such a pair element is called key and the tail is called value.

S-Expressions

A symbolic expression (S-expression) is a data structure defined as: An S-expression is either an atom or a pair of S-expressions. In Lisp, Scheme, and in particular in SLED, S-expressions are used for data as well as source code.

Numbers

The SLED system does not feature numeric types. Yet natural numbers (non-negative integers) can be emulated using lists:

'()             ; zero
'(nil)          ; one
'(nil nil)      ; two
'(nil nil nil)  ; three

These are tally numerals, so cardinality represents the magnitude, which is similar to von Neumann ordinals. The functions inc, dec, and zero? facilitate with counting tasks.

Code

In LISP, unquoted data is evaluated as code.

Expressions

Expressions can be evaluated, like:

• Bindings
• Functions
• Special Forms

Bindings

A binding links a symbol to some data payload, and is created via the define special form:

(define a 'x)

Closures

Closures are functions together with an environment, and result from the lambda special form:

(define fun (lambda (arg1 arg2) (print arg1) (print arg2)))

Function Application

The first element of an unquoted list is interpreted as an expression that evaluates to a function and the remaining elements as arguments to that function:

(fun arg1 arg2)

The function evaluation is eager; so first, the argument expressions are evaluated, then the function application is using the evaluated arguments.

Arguments

Arguments are evaluated and passed as a list of values to a function.

This means the function parameters can be set up in various ways:

(lambda x ...)          ; x is a list
(lambda (x y) ...)      ; destructured list with elements x and y
(lambda (x y . z) ...)  ; z is a list (which is by default nil)

Optional arguments can be passed as a list, like z above.

Recursion

LISPs rely on recursion instead of iteration. Recursion refers to a function calling itself. Two features of SLED help avoid a stack overflow in deep recursions: the trampoline evaluator and tail-call optimization (TCO). TCO works for lambda, let, begin, if, ifnil, and apply. Additionally, TCO works for cons if the recursion runs in the second argument.

Errors

An error during evaluation of an expression jumps back to the top-level, where an error occurrence can be tested. An error cannot be caught inside an expression.

Builtin Functions

A set of functions is built into the SLED executable to enable interaction with the system and core functionality, for details see the index.

Standard Library

Beyond the core functions a set of typical functions is implemented as a standard library in the file sled.scm. For details see the index. The standard library may be extended with additional custom definitions.

Special Forms

Certain forms appear like functions but are not. These so-called special forms do not follow the function behavior, but use the same syntax as functions. For example, if does not evaluate its arguments before resolving the form. For details see the index.

Immutability

Special symbols, special forms, builtin functions, and standard library contents are immutable in SLED. Furthermore, special forms and builtin functions cannot be shadowed. Standard library and adjacent custom definitions cannot be redefined.

System

This LISP system is a DOS application.

File Names

File names should follow DOS 8.3 naming (maximum 8 characters for the file name, a dot, maximum 3 characters for the file extension). The recommended file extension for scripts running on this LISP is .scm due to syntactic similarity to Scheme; for instance, the standard library is named sled.scm. However, the interpreter does not check the file extension.

Startup

The first action sled takes is loading its standard library, which has to have the name sled.scm and is expected in the same directory as the SLED.EXE interpreter executable. All symbols and their values loaded from the standard library become immutable.

Command-Line Arguments

The sled binary has four mutually exclusive command-line arguments. The first just displays a help page:

C:\> sled /?

The second is a path to a Lisp source file to be loaded before the REPL starts, but after the standard library loaded:

C:\> sled code.scm

The third is a "batch mode" switch /B, which exits after execution:

C:\> sled /B code.scm

The fourth is an "ignore errors" switch /I, which behaves like /B but continues execution after an error occurs:

C:\> sled /I code.scm

The file path has to be the last argument.

File Path

In the interpreter, if the path contains backslashes these need to be escaped, since the path becomes a symbol and the backslash \ alone is not an admissible symbol character.

C:\> sled to\my\code.scm

(load 'to\\my\\code.scm)

The file path also falls under the 16 character limit.

REPL

Once sled started, the read-eval-print-loop (REPL) begins with a prompt:

sled>

It reads input, evaluates it, prints the result, and prompts again.

Now have fun:

sled> (println 'hello _ 'world)

Extended Characters

A pitfall is extended (two-byte) characters which are not supported. An example is using the arrow keys in the REPL, resulting in an α (alpha) in the standard input echo. These extended characters pollute the input stream and can cause an error in an input line even if deleted.

Exiting

There are two regular ways to exit sled.

The first is the dollar symbol $ on the top-level, which tells the parser to exit:

sled> $

The second is the (exit) builtin function, which upon evaluation exits:

(exit)

Comments

Comments are ignored by the parser. A comment is introduced by a semicolon:

;

All characters until the next line break are ignored by the parser. Traditionally, the number of consecutive semicolons conveys semantics, similar to Markdown headings:

;;;; main title

;;; section title

;; start of line

; end of line

Furthermore, block comments are realized via a special form named comment:

(comment ...)

Parentheses inside a comment form need to be balanced:

(comment ())  ; OK
(comment ()   ; → error

Note that a comment form cannot be quoted:

'(comment test)  ; → error

Breaking

To break pure computation, use CTRL+Break, for breaking input CTRL+C is available.

Help

In the REPL, the symbol ? can be used to list special forms, builtin functions, and standard library symbols.

Limits

As real-mode DOS program, SLED has multiple constraints:

• The heap has 12288 nodes
• The symbol table has 2048 characters

The standard library consumes about 5% of nodes and characters.

Index

Special Symbols Special Forms Builtin Functions Standard Aliases Standard Library

---

$ {exit}

This special symbol exits the REPL. This is not a short form of (exit), as it is resolved by the parser. Works only from the prompt.

---

? {help}

This special symbol holds an overview of special forms, builtin functions, and the standard library. Works only from the prompt.

---

' {quote}

This special symbol is an alias for the quote special form.

---

_ {space}

This standard alias is for the space character '\ .

---

and? <arg1> <arg2>

This standard library binary predicate answers if both arguments are not nil; use to make compound conditionals simpler.

(and? nil nil)    ; → nil
(and? nil true)   ; → nil
(and? true nil)   ; → nil
(and? true true)  ; → true

NOTE: Unlike in Scheme, this is not a special form of variadic arguments which are evaluated sequentially until a false result, but a binary function that evaluates both arguments.

---

ans

This special symbol contains the result of the last top-level form that produced a value; in case of an error, the err symbol is set, which can be tested for with error?.

ans

---

append <lst1> <lst2>

This standard library binary function returns a list consisting of the second argument list concatenated to the end of the first argument list.

(append nil (list 'a))        ; → (a)
(append (list 'a) nil)        ; → (a)
(append (list 'a) (list 'b))  ; → (a b)
(append '(a b) '(c d))        ; → (a b c d)
(append '(a) 'b)              ; → (a . b)

---

apply <fun> <lst>

This builtin binary function evaluates the first argument function with the second argument list as arguments.

(apply list '(a b c))  ; → (a b c)

---

assert <arg> <sym>

This standard library binary procedure prints the second argument and causes an error, if the first argument evaluates to nil.

(assert nil 'list\ empty)

---

atom? <arg>

This builtin unary predicate answers if the argument is an atom.

(atom? nil)   ; → true
(atom? true)  ; → true
(atom? '(a))  ; → nil

---

begin <body1> ... <bodyN>

This special form evaluates its arguments in sequence and returns the last argument's return value.

(begin (print 'a) (print 'b) 'c)  ; → c
(begin)                           ; → nil

---

br

This standard alias is for newline.

---

This special form is not evaluated. Use as block comment. Unlike the other special forms, this is decoded by the parser before evaluation.

(comment this :-D is ignored)

NOTE: Parentheses inside comments have to be balanced.

---

compose <arg> <fun1> ... <funN>

This standard library variadic function pipelines unary functions: the second argument is applied to the first argument, the third argument is then applied to the previous return value and so on; the final argument's return value is returned as result.

(compose nil inc inc)  ; → (nil nil)

---

cons <arg1> <arg2>

This builtin binary function returns a pair with the first argument as head and second argument as tail.

(cons 'a 'b)  ; → (a . b)

---

dec <arg>

This standard library unary function returns the tail of a list if not empty; use to decrement von tally numbers.

(dec '(nil . nil))  ; → nil

---

define <sym> <arg>

This special form creates a new binding of the second argument to the first argument symbol, and returns the second argument. Bindings can be re-defined, except special symbols, special forms, builtin functions, or standard library symbols.

(define hello 'world)  ; → world

NOTE: define always affects the global binding, also when used inside let or lambda.

---

defined? <sym>

This builtin unary predicate answers if the argument symbol is already defined at global scope.

(defined? 'defined?)   ; → true
(defined? 'undefined)  ; → nil

---

empty? <arg>

This builtin unary predicate answers if its argument is the empty list.

(empty? nil)  ; → true

---

env

This builtin thunk prints the current user-defined symbols.

(env)

---

eof? <arg>

This builtin unary predicate answers if its argument evaluates to an end-of-file (EOF) or end-of-transmission (EOT) symbol.

(ifnil (eof? (read)) 'none)

---

equal? <arg1> <arg2>

This standard library binary predicate answers if the arguments are recursively equal. Use to compare pairs and lists, however equal? falls back to equiv? for atoms.

(equal? '(nil nil) (list nil nil))  ; → true
(equal? nil nil)                    ; → true

---

equiv? <arg1> <arg2>

This builtin binary predicate answers if the arguments are shallowly equal. Use to compare atoms.

(equiv? nil nil)  ; → true

---

err

This special symbol marks an error state.

(head nil) (equiv? err ans)  ; → true

---

error <sym> [<arg>]

This builtin procedure throws an error and thus causes a break in evaluation of the current form. Furthermore, the first argument symbol (error message) and the optional second argument expression (error reason) are printed.

(error 'bad\ error (list 'not 'right))

---

error?

This standard library thunk predicate answers if the previous evaluation resulted in an error.

(head nil) (error?)  ; → true

---

exit

This builtin thunk quits the interpreter or REPL.

(exit)  ; back to DOS

---

gc [<arg>]

This builtin procedure triggers garbage collection. Node usage is printed if an argument is provided which is not nil.

(gc)       ; → *no output*
(gc nil)   ; → *no output*
(gc true)  ; → *prints node usage*

---

get <sym> <lst>

This standard library binary function returns the value paired to the first argument symbol if found in the second argument association list, or nil otherwise.

(get 'a '((a . x) (b . y)))  ; → x
(get 'z '((a . x) (b . y)))  ; → nil

NOTE: Uses equiv? for key comparisons.

---

head <arg>

This builtin unary function returns the head part of a cons cell or list.

(head (cons 'a 'b))  ; → a

NOTE: This function corresponds to car in classic LISP and Scheme.

---

id <arg>

This standard library unary function returns its argument. Use as identity function.

(id 'x)  ; → x

---

if <arg1> <arg2> [<arg3>]

This special form evaluates the first argument; if it does not evaluate to nil, the second argument is evaluated and returned, otherwise the third argument is evaluated and returned, or `nil` if no third argument is given.

(if 'ok 'con 'alt)  ; → con
(if nil 'con 'alt)  ; → alt
(if 'ok 'con)       ; → con
(if nil 'con)       ; →

---

ifnil <arg1> <arg2>

This special form evaluates the first argument and returns its result if it is not nil, otherwise the second argument is evaluated and returned.

(ifnil 'ok 'alt)  ; → ok
(ifnil nil 'alt)  ; → alt

---

inc <arg>

This standard library unary function prepends nil to a list. Use to increment tally numbers.

(inc nil)     ; → (nil)
(inc '(nil))  ; → (nil nil)

---

lambda (<arg1> ... <argN>) <body1> ... <bodyN>

This special form creates a function with arguments as destructured list and a sequentially evaluated body, whose last expression is the return value.

(lambda (x y) (print x) y)  ; → function that prints x and returns y

---

let (<arg1> <arg2>) <body1> ... <bodyN>

This special form creates a scope with a local binding and evaluates its body sequentially.

(let (x 'y) x)  ; → y

NOTE: Unlike Scheme, this special form allows only a single local binding. Multiple bindings can be realized by nesting lets.

---

list <arg1> ... <argN>

This standard library variadic function constructs a list of its arguments.

(list)        ; → nil
(list 'a)     ; → (a)
(list 'a 'b)  ; → (a b)

---

list? <arg>

This standard library unary predicate answers if the argument is a proper list.

(list? nil)     ; → true
(list? true)    ; → nil
(list? '(x y))  ; → true

---

load <sym> [<arg>]

This builtin function evaluates the contents of the file given by the argument symbol (path) and returns the last answer. If a second argument, which is not nil, is given then errors are ignored during loading.

(load 'myscript.scm)
(load 'myscript.scm true)

NOTE: Loads can be nested twice at most.

---

map <fun> <lst>

This standard library binary function applies the first argument unary function to each element of the second argument list and returns the list of return values.

(map inc '(nil (nil)))  ; → ((nil) (nil nil))

---

member <arg> <lst>

This standard library binary function returns the pair from the second argument list whose head is the first argument; otherwise nil is returned.

(member 'b '(a b c))  ; → (b c)
(member 'd '(a b c))  ; → nil

NOTE: Uses equiv? for list element comparisons.

---

newline

This builtin thunk prints a line break; use for output formatting.

(newline)

---

nil

This special symbol represents the empty list; and is also the only value evaluating to "false". Equivalently, '() can be used for nil.

nil  ; → nil
'()  ; → nil

---

nil? <arg>

This is a standard alias for empty?; use as test for nil.

(nil? nil)     ; → true
(nil? '())     ; → true
(nil? _)       ; → nil
(nil? (list))  ; → true

---

not <arg>

This is a standard alias for empty?; use for inverting predicate results.

(not nil)   ; → true
(not true)  ; → nil

---

or? <arg1> <arg2>

This standard library binary predicate answers if any argument is not nil; use to simplify compound conditionals.

(or? nil nil)    ; → nil
(or? nil true)   ; → true
(or? true nil)   ; → true
(or? true true)  ; → true

NOTE: Unlike in Scheme, this is not a special form of variadic arguments which are evaluated sequentially until a true result, but a binary function that evaluates both arguments.

---

pair? <arg>

This standard library unary predicate answers if the argument is not an atom.

(pair? 'a)      ; → nil
(pair? '(a b))  ; → true
(pair? nil)     ; → nil

---

print <arg1> ... <argN>

This builtin variadic procedure prints its arguments to the standard output.

(print 'hi)
(print 'hello _ 'world)
(print '(a b))

---

printid <arg1> [<arg2>]

This standard library function returns the first argument after printing it and a line break; if given, the second argument is printed before the first.

(printid ans 'answer)

---

println <arg1> ... <argN>

This standard library variadic procedure prints its arguments to the standard output and appends a line break.

(println 'hi)
(println 'hello _ 'world)
(println '(a b))

---

proc? <arg>

This builtin unary predicate answers if its argument is a closure or builtin function. For special forms, this predicate returns nil.

(proc? map)    ; → true
(proc? proc?)  ; → true
(proc? nil)    ; → nil
(proc? if)     ; → nil

---

put <sym> <arg> <lst>

This standard library function returns an updated third argument association list by setting the tail of the pair with the first argument symbol as head, or adding a pair with the first argument symbol as head and the second argument as tail, if no pair with the first argument symbol as head is listed in the third argument.

(put 'hello 'world nil)               ; → ((hello . world))
(put 'one 'uno '((one . a)))          ; → ((one . uno))
(put 'two 'b '((one . a)))            ; → ((one . a) (two . b))
(put 'two 'z '((one . a) (two . b)))  ; → ((one . a) (two . z))

---

quit

This standard alias is for exit.

---

quote <arg>

This special form returns its argument unevaluated.

(quote a)  ; → a
'a         ; → a

---

read

This builtin thunk returns a symbol read as a line of input from the standard input source terminated by a line break via return key / enter key.

(read)

---

restart [<sym>]

This builtin function resets and restarts the interpreter and optionally loads a file specified by the symbol argument. All definitions are lost!

(restart 'next.scm)

---

reverse <lst>

This standard library unary function reverses its list argument.

(reverse (list 'a 'b 'c))  ; → (c b a)

---

self <arg1> ... <argN>

This special symbol enables anonymous recursion. Inside any closure it resolves to the enclosing lambda. Outside a lambda, self is undefined.

((lambda (x)
  (if (empty? x) nil
                 (self (tail x)))) (list nil nil))  ; → nil

---

shorter? <lst1> <lst2>

This standard library binary predicate answers if the first argument list has less elements than the second argument list.

(shorter? nil (list nil))  ; → true
(shorter? (list nil) nil)  ; → nil

---

symbol? <arg>

This builtin unary predicate answers if its argument is a symbol.

(symbol? 'a)  ; → true

---

tail <arg>

This builtin unary function returns the tail part of a cons cell.

(tail (cons 'a 'b))  ; → b

NOTE: This function corresponds to cdr in classic LISP and Scheme.

---

true

This special symbol evaluates to true. Use as a generic "true" value.

true  ; → true

---

value <sym>

This builtin unary function resolves the value of its symbol argument. Respects lexical scope.

(value 'ver)  ; → sled-0.4

---

ver

This special symbol evaluates to a symbol pinpointing the version of SLED.

ver  ; → sled-0.4

---

zero? <arg>

This is a standard alias for empty?; use for testing tally numbers.

(zero? nil)     ; → true
(zero? '(nil))  ; → nil